Quick update: we're in a park here in Iowa where 4 campers are sharing the same hose bib. In this setting others are affecting our pressure. Additionally the park has fair to low pressure on its own. We're not experiencing any issue with the On Demand system. Our flow is higher than the pump, but not high enough to wash down anything. Even at 3gpm the unit worked well, here with the low to lower pressure, when the neighbor runs water, we're still getting a constant flow of hot water. The only people who ever expressed concern or negativity about RV On Demand systems were people who did not have one.
Is there a drawback? Our grey tank is probably filling faster as our showers are longer but again we don't boondock nor are we without hook ups. We move every two weeks.
Still very happy.